Ultrasonic diagnostic apparatus and method for processing ultrasonic signal

ABSTRACT

The invention provides an ultrasonic diagnostic apparatus for acquiring images different in a frequency, and the ultrasonic diagnostic apparatus according to the invention is composed of a transducer including plural elements for sending an ultrasonic wave and receiving the reflected ultrasonic wave, analog to digital converters that digitize plural received signals, first mixers that respectively multiply a signal output from the analog to digital converter and a first digital reference signal, first filters that respectively extract a signal having a predetermined center frequency from a signal output from each first mixer, digital delay units that respectively delay a signal output from each first filter, an adder that adds plural signals output from the digital delay units, a second mixer that multiplies a signal output from the adder and a second digital reference signal, an envelope detector that detects a signal output from the second mixer, a scan converter that converts a signal output from the envelope detector to a picture signal and a display that displays a signal output from the scan converter. Hereby, the pass band of the filter is not required to be changed.

TECHNICAL FIELD

[0001] The present invention relates to an ultrasonic diagnosticapparatus, particularly relates to an ultrasonic diagnostic apparatusfor nondestructive inspection that executes processing for converting anultrasonic signal to a digital signal and an ultrasonic diagnosticapparatus for medicine.

BACKGROUND ART

[0002]FIG. 2 shows an example of the configuration of an ultrasonicdiagnostic apparatus according to prior art using a digital circuit. Anultrasonic signal s (t) sent from a transducer 1 composed of pluralelements and having a mean frequency ω_(s) can be approximatelyexpressed by an expression 1. “A (t)” denotes the shape of an envelopeof the sent signal, “t” denotes a variable of time and “j” denotes animaginary unit.

s(t)=A(t)×{exp(jω_(s) t)+exp(−jω_(s) t)}  Expression 1

[0003] The transducer 1 receives an ultrasonic wave reflected on anobject of inspection. A signal f_(n) (t) received by an “n”th element ofthe transducer 1 is expressed by an expression 2 in case propagationdelay time from the sending of an ultrasonic wave to the receiving of itis τ_(n). “φ_(n)” denotes a phase of the received signal f_(n) (t) andis acquired in an expression 3. $\begin{matrix}\begin{matrix}{{f_{n}(t)} = {s( {t - \tau_{n}} )}} \\{= {{A( {t - \tau_{n}} )} \times \{ {{\exp \lbrack {j( {{\omega_{s}t} - \varphi_{n}} )} \rbrack} + {\exp \lbrack {- {j( {{\omega_{s}t} - \varphi_{n}} )}} \rbrack}} \}}}\end{matrix} & {{Expression}\quad 2}\end{matrix}$

 φ_(n)=ω_(s)τ_(n) Expression 3

[0004] When the number of elements used for one sending/receiving of anultrasonic wave is N, N pieces of output signals from the transducer 1are respectively expressed by the expression 2. The propagation delaytime τ_(n) is different for every element. In the following description,element n used for a series of one sending/receiving of an ultrasonicwave are called channel n and “N” denotes the number of channels. InFIGS. 1, 2 and 6, a selecting circuit and a driving circuit for Nelements used for a series of sending/receiving of an ultrasonic waveare omitted and are not shown.

[0005] An analog to digital converter 2 converts the received signalf_(n) (t) to digital data and the succeeding signal processing is alldigital signal processing. The precision of operation is enhanced bydigital signal processing, compared with that in analog signalprocessing. An A/D converter is generally used for the analog to digitalconversion part 2.

[0006] A mixer 3 multiplies the received signal f_(n) (t) converted todigital data and a digital reference signal h_(n) (t) expressed by anexpression 4. A product g_(n) (t) of multiplication is expressed by anexpression 5. “h_(n) (t)” has the same frequency as the center frequencyω_(s) of the received signal. $\begin{matrix}\begin{matrix}{{h_{n}(t)} = {\exp ( {{j\omega}_{s}t} )}} \\{{g_{n}(t)} = {{f_{n}(t)}{h_{n}(t)}}}\end{matrix} & {{Expression}\quad 4} \\{\quad {= {{A( {t - \tau_{n}} )} \times \{ {{\exp \lbrack {j( {{2\omega_{s}t} - \varphi_{n}} )} \rbrack} + {\exp ( {j\quad \varphi_{n}} )}} }}} & {{Expression}\quad 5}\end{matrix}$

[0007] Next, a filter 4 extracts a low-frequency component from theproduct of multiplication (the expression 5). The product of themultiplication from which the low-frequency component is extracted isexpressed by an expression 6. The filter 4 is formed by an accumulatorand an element for computing the sum of products for example.

g _(n) (t)=A(t−τ_(n))×exp(jφ_(n))  Expression 6

[0008] A digital delay unit 5 multiplies a signal acquired by delaying asignal output from the filter 4 and expressed by the expression 6 byτ_(n) by exp (−j φ_(n). A signal V_(n) (t) output from the digital delayunit 5 is expressed by an expression 7. The output signal V_(n) (t) isfixed independently of a channel without depending upon n.$\begin{matrix}{{V_{n}(t)} = {{{g_{n}( {t + \tau_{n}} )} \times {\exp ( {{- j}\quad \varphi_{n}} )}} = {A(t)}}} & {{Expression}\quad 7}\end{matrix}$

[0009] The output signal V_(n) (t) output from the digital delay unit 5is added in an adder 6 by the number (N) of all channels of a series ofelement n (channel n) used for a series of sending/receiving of anultrasonic wave. A result of addition grows N times of a signal of asingle channel, if a phase of each channel is coincident. N signal linesfrom the transducer 1 to the adder 6 are converted to a signal line inthe adder 6.

[0010] In the above description, as each processing in the analog todigital converter 2, the mixer 3, the filter 4 and the digital delayunit 5 is executed every channel, parallel N pieces of respective unitsare required.

[0011] In this process, signals received from directions except adesired direction vanish because they have different phases. Signalprocessing described above is generally called beam forming. Ultrasonicbeams can be formed in the desired direction by the beam forming.

[0012] For documents related to the beam forming addition, there areJapanese patent No. 1333370 and U.S. Pat. No. 4,140,022 and U.S. Pat.No. 4983970.

[0013] In an envelope detector 7, the absolute value of “N×A (t)” whichis a signal output from the adder 6 is acquired and a scan converter 8applies signal processing such as the compression of a logarithm andgamma conversion to a signal output from the envelope detector 7. Asignal output from the scan converter 8 is displayed on a display 9 as atomographic image of an object to be inspected. In the expression 7, acomplex number is generally acquired and the envelope detector 7calculates the absolute value of the complex number (the root square sumof the real part and the imaginary part).

[0014] In the ultrasonic diagnostic apparatus according to the prior artshown in FIG. 2, the center frequency of the received signal to beimaged is required to be determined beforehand. In the conventional typeapparatus shown in FIG. 2, the center frequency of the received signalto be imaged is the same ω_(s) as the center frequency of the sentsignal and is equal to the frequency of the digital reference signalh_(n) (t). The ultrasonic diagnostic apparatus according to the priorart shown in FIG. 2 has the problem that the frequency of the digitalreference signal h_(n) (t) is required to be equalized to the centerfrequency of the received signal to be imaged and the center frequencyof the received signal to be imaged is limited to the predetermined onefrequency.

[0015] The ultrasonic diagnostic apparatus according to the prior artshown in FIG. 2 has the problem that as the filter 4 removes thehigh-frequency component (2ω_(s)) which is an unnecessary component fromthe signal output from the mixer 3, the frequency of the unnecessaryhigh-frequency component also varies in case the center frequency ω_(s)of the received signal to be imaged is varied and the pass band of thefilter 4 is required to be varied in accordance with the centerfrequency of the received signal to be imaged.

[0016] There is the problem that as the filter 4 is required for everychannel, the scale of the apparatus is enlarged when the configurationof the filter 4 is complex and the apparatus becomes high-priced.

DISCLOSURE OF THE INVENTION

[0017] The object of the invention is to provide an ultrasonicdiagnostic apparatus in which plural images can be formed based uponreceived signals different in the center frequency without changing thepass band of the filter.

[0018] Another object of the invention is to provide an ultrasonicdiagnostic apparatus in which signal processing for acquiring pluralimages based upon received signals different in the center frequency canbe simultaneously executed in parallel and the pass band of the filterof each channel is not required to be varied even if the centerfrequencies of received signals to be imaged are varied.

[0019] To achieve the objects, the ultrasonic diagnostic apparatusaccording to the invention is provided with the following configuration.

[0020] In the first configuration of the invention, plural receivedsignals received by a transducer composed of plural elements for sendingan ultrasonic wave to an object to be inspected and receiving theultrasonic wave reflected from the object to be inspected are digitizedin an analog to digital converter. A first mixer multiplies a signaloutput from the analog to digital converter and a first digitalreference signal.

[0021] The first filter extracts a signal having a predetermined centerfrequency from the signal output from the first mixer. A digital delayunit delays the signal output from the first filter and an adder addsplural signals output from the digital delay unit.

[0022] The second mixer multiplies the signal output from the adder andthe second digital reference signal. An envelope detector detects thesignal output from the second mixer, a scan converter converts thesignal output from the envelope detector to a picture signal and adisplay displays the signal output from the scan converter.

[0023] In the first configuration, the sum of the frequency of the firstdigital reference signal and the frequency of the second digitalreference signal is equalized to the center frequency of the receivedsignal to be imaged.

[0024] In the second configuration of the invention, plural signalprocessing circuits each of which is composed of the second mixer thatmultiplies the signal output from the adder and the second digitalreference signal, the second filter that extracts a signal having apredetermined center frequency from a signal output from the secondmixer, an envelope detector that detects the signal output from thesecond filter and a scan converter that converts the signal output fromthe envelope detector to a picture signal are connected to an outputterminal of the adder in the first configuration of the invention inparallel. A display that displays the signal output from the scanconverter may be also provided to each signal processing circuit.

[0025] In the second configuration, in each signal processing circuit,the second filter extracts a signal having a predetermined centerfrequency different for every signal processing circuit from the signaloutput from the second mixer and the envelope detector detects thesignal output from the second filter.

[0026] The sum of the frequency of the first digital reference signaland the frequency of the second digital reference signal is equalized tothe center frequency of the received signal to be imaged. Signalprocessing for acquiring images the respective center frequencies ofwhich are different is simultaneously executed in parallel. Images basedupon received signals respectively having a different center frequencyare displayed on a display of each signal processing circuit or the samesingle display.

[0027] In the third configuration of the invention, first and secondsignal processing circuits each of which is composed of the second mixerthat multiplies a signal output from the adder and the second digitalreference signal, the second filter that extracts a signal having apredetermined center frequency from the signal output from the secondmixer, an envelope detector that detects the signal output from thesecond filter and a scan converter that converts the signal output fromthe envelope detector to a picture signal are connected to the outputterminal of the adder in the first configuration of the invention inparallel. A display that displays the signal output from the scanconverter may be also provided to the first and second signal processingcircuits.

[0028] In the third configuration, in the first and second signalprocessing circuits, the second filter extracts a signal having apredetermined center frequency different between the first and secondsignal processing circuits from the signal output from the second mixerand the envelope detector detects a signal output from the secondfilter. Signal processing for acquiring images of the first and secondcenter frequencies is simultaneously executed in parallel.

[0029] The frequency of the first digital reference signal is setbetween the center frequency of the first received signal and the centerfrequency of the second received signal and desirably, is set to theirmean value. An image based upon a received signal having the firstcenter frequency is displayed on a display of the first signalprocessing circuit and an image based upon a received signal having thesecond center frequency is displayed on a display of the second signalprocessing circuit. Or images based upon received signals having thefirst and second center frequencies are displayed on the same singledisplay.

[0030] The digital ultrasonic diagnostic apparatus according to theinvention is characterized in that received signals having differentcenter frequencies can be imaged without changing the pass band of thefilter.

[0031] Also, the digital ultrasonic diagnostic apparatus according tothe invention is characterized in that signal processing for acquiringplural images the respective center frequencies of which are differentcan be simultaneously executed in parallel without changing theconfiguration including the analog to digital converter 2, the mixer 3,the filter 4 and the digital delay unit 5 respectively required everychannel of the ultrasonic diagnostic apparatus according to the priorart shown in FIG. 2 and even if the center frequency of a receivedsignal to be imaged is varied, the pass band of the filter every channelis not required to be varied.

[0032] Further, the invention provides an ultrasonic diagnosticapparatus provided with a transducer composed of plural elements, aselecting/driving circuit for sending an ultrasonic wave having a centerfrequency ω_(s) to an object to be inspected and selecting plural (npieces of) elements (n=1, 2, - - - , N) that receive the ultrasonic wavereflected from the object to be inspected and having the centerfrequency ω_(s), an analog to digital converter provided correspondingto each of the plural (n pieces of) elements (n=1, 2, - - - , N) fordigitizing a signal received by the element, a first mixer providedcorresponding to the analog to digital converter corresponding to eachelement for multiplying a signal output from the analog to digitalconverter and the first digital reference signal having a frequencyω_(m) different from the center frequency ω_(s), the first filterprovided corresponding to the first mixer corresponding to each elementfor extracting a signal having a frequency (ω_(m)−ω_(s)) from a signaloutput from the first mixer, a digital delay unit provided correspondingto the first filter corresponding to each element for multiplying asignal output from the first filter and delayed by propagation delaytime τ_(n) different every element from the sending of the ultrasonicwave to the receiving of it by “exp (−jω_(m)τ_(n))”, an adder that addsrespective signals output from the digital delay units corresponding tothe plural (n pieces of) elements (n=1, 2, - - - , N), the second mixerthat multiplies a signal output from the adder and a second digitalreference signal having a frequency (ω_(s)−ω_(m)) , an envelope detectorthat detects the signal output from the second mixer, a scan converterthat converts the signal output from the envelope detector to a picturesignal and a display that displays the signal output from the scanconverter and characterized in that a received signal having the centerfrequency ω_(s) is imaged and displayed.

[0033] Furthermore, the invention provides a method of processing anultrasonic signal provided with the following processes;

[0034] (1) a process for selecting and driving the plural (n pieces of)elements (n=1, 2, - - - , N) of the transducer composed of pluralelements for sending an ultrasonic wave having a center frequency ω_(s)to an object to be inspected and receiving the ultrasonic wave reflectedfrom the object to be inspected and having the center frequency ω_(s);

[0035] (2) a process for digitizing the plural received signals receivedby each element in the analog to digital converters providedcorresponding to the plural (n pieces of) elements (n=1, 2, - - - , N);

[0036] (3) a process for multiplying a signal output from the analog todigital converter and a first digital reference signal having afrequency ω_(m) different from the center frequency ω_(s) in the firstmixer provided corresponding to the analog to digital convertercorresponding to each element;

[0037] (4) a process for extracting a signal having a frequency(ω_(m)−ω_(s)) from a signal output from the first mixer in the firstfilter provided corresponding to the first mixer corresponding to eachelement;

[0038] (5) a process for multiplying a signal output from the firstfilter and delayed by propagation delay time τ_(n) different for everyelement from the sending of the ultrasonic wave to the receiving of itby “exp (−jω_(m)τ_(n))” in the digital delay unit provided correspondingto the first filter corresponding to each element;

[0039] (6) a process for adding signals output from the digital delayunits corresponding to the plural (n pieces of) elements (n=1, 2, - - -, N) in the adder;

[0040] (7) a process for multiplying a signal output from the adder andthe second digital reference signal having a frequency (ω_(s)−ω_(m)) inthe second mixer;

[0041] (8) a process for detecting a signal output from the second mixerin the envelope detector;

[0042] (9) a process for converting a signal output from the envelopedetector to a picture signal in the scan converter; and

[0043] (10) a process for displaying a signal output from the scanconverter on the display and characterized in that a received signalhaving the center frequency ω_(s) is imaged and displayed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a block diagram showing an example of the configurationof an ultrasonic diagnostic apparatus equivalent to the first embodimentof the invention;

[0045]FIG. 2 is a block diagram showing an example of the configurationof an ultrasonic diagnostic apparatus according to prior art using adigital circuit;

[0046]FIGS. 3a to 3 c are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus according to the prior art shown in FIG. 2;

[0047]FIGS. 4a to 4 c are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus according to the prior art shown in FIG. 2;

[0048]FIGS. 5a to 5 d are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus equivalent to the first embodiment of the invention;

[0049]FIG. 6 is a block diagram showing an example of the configurationof an ultrasonic diagnostic apparatus equivalent to the secondembodiment of the invention;

[0050]FIGS. 7a to 7 c are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus equivalent to the second embodiment of the invention;

[0051] and FIGS. 8a to 8 d are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus equivalent to the second embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] Referring to the drawings, embodiments of the invention will bedescribed in detail below. In the following embodiments, an ultrasonicdiagnostic apparatus for medicine in which the digital signal processingof an ultrasonic signal is executed is described for example, however,the invention can be similarly applied to an ultrasonic apparatus fornondestructive inspection.

First Embodiment

[0053]FIG. 1 shows an example of the configuration of an ultrasonicdiagnostic apparatus equivalent to the first embodiment of theinvention. As shown in FIG. 1, the configuration except the first mixer10 and the second mixer 11 is the same as the configuration of theultrasonic diagnostic apparatus according to the prior art shown in FIG.2. The signal f_(n) (t) received by the “n”th element of the transducer1 due to an ultrasonic signal s (t) sent from the transducer 1 andhaving a center frequency a is expressed by an expression 8 whenpropagation delay time from the sending of an ultrasonic wave to thereceiving of it is τ_(n) as in the expression 2. “φ_(n)” denotes a phaseof the received signal f_(n) (t) and is expressed by an expression 9 asin the expression 3.

[0054] In case the number of elements used for one sending/receiving ofan ultrasonic wave is N, N pieces of output signals output from thetransducer 1 are respectively expressed by the expression 8. Thepropagation delay time τ_(n) is different every element. $\begin{matrix}\begin{matrix}{{f_{n}(t)} = {s( {t - \tau_{n}} )}} \\{= {{A( {t - \tau_{n}} )} \times \{ {{\exp \lbrack {j( {{\omega_{s}t} - \varphi_{n}} )} \rbrack} + {\exp \lbrack {- {j( {{\omega_{s}t} - \varphi_{n}} )}} \rbrack}} \}}}\end{matrix} & {{Expression}\quad 8} \\{{\varphi_{n} = {\omega_{s}\tau_{n}}}\quad} & {{Expression}\quad 9}\end{matrix}$

[0055] The received signal f_(n) (t) is converted to digital data bythen analog to digital converter (an A/D converter) 2 and the succeedingsignal processing is all digital signal processing using the expression8 in which a discrete value is acquired.

[0056] The first mixer 10 multiplies the received signal f_(n) (t)converted to digital data and a digital reference signal h_(n) (t)expressed by an expression 10. A product g_(n) (t) of multiplication isexpressed by an expression 11. In the invention, a frequency of thedigital reference signal h_(n) (t) and center frequencies ω_(s) ofreceived signals can be differentiated. In case a frequency of h_(n) (t)is ω_(m), h_(n) (t) is as follows. $\begin{matrix}{\begin{matrix}{{h_{n}(t)} = {\exp ( {{j\omega}_{m}t} )}} \\{{g_{n}(t)} = {{f_{n}(t)}{h_{n}(t)}}}\end{matrix}\quad} & {{Expression}\quad 10} \\{\quad {= {{A( {t - \tau_{n}} )} \times \{ {{\exp \lbrack {j( {{( {\omega_{s} + \omega_{m}} )t} - \varphi_{n}} )} \rbrack} + {\exp \lbrack {- {j( {{( {\omega_{s} - \omega_{m}} )t} - \varphi_{n}} )}} \rbrack}} \}}}} & {{Expression}\quad 11}\end{matrix}$

[0057] Next, the filter 4 extracts a low-frequency component from theproduct (the expression 11) of the multiplication. The product of themultiplication from which the low-frequency component is extracted isexpressed by the expression 12. The filter 4 is formed by an accumulatorand an element for computing the sum of products as in the prior art.

g _(n)(t)=A(t−τ_(n))×exp{−j[(ω_(s)−ω_(m))t−φ_(n)]}  Expression 12

[0058] The digital delay unit 5 multiplies a signal acquired by delayingthe signal (the expression 12) output from the filter 4 by τ_(n) by exp(−ω_(m)τ_(n)). A signal V_(n) (t) output from the digital delay unit 5is expressed by the expression 13. In the invention, a complex numbermultiplied in the digital delay unit 5 is not “φ_(n)=ω_(s) τ_(n)” butthe product ω_(m) τ_(n). The output signal V_(n) (t) is fixedindependent of the channel number n. $\begin{matrix}\begin{matrix}{{V_{n}(t)} = {{g_{n}( {t + \tau_{n}} )}{\exp ( {{- j}\quad \omega_{m}\tau_{n}} )}}} \\{= {{A(t)} \times \exp \{ {- {j\lbrack {{( {\omega_{s} - \omega_{m}} )( {t + \tau_{n}} )} - \varphi_{n} + {\omega_{m}\tau_{n}}} \rbrack}} \}}} \\{= {{A(t)} \times {\exp ( {- {j\lbrack {{( {\omega_{s} - \omega_{m}} )t} + {\omega_{s}\tau_{n}} - {\omega_{m}\tau_{n}} - {\omega_{s}\tau_{n}} + {\omega_{m}\tau_{n}}} \rbrack}} )}}} \\{= {{A(t)} \times \exp \{ {{- {j( {\omega_{s} - \omega_{m}} )}}t} \}}}\end{matrix} & {{Expression}\quad 13}\end{matrix}$

[0059] The adder 6 adds the output signal V_(n) (t) output from thedigital delay unit 5 in respect to element (channel) number n used forone sending/receiving of an ultrasonic wave for all N channels and theresult of addition is expressed by the expression 14.

S(t)=N×A(t)×exp{−j(ω_(s)−ω_(m))t}  Expression 14

[0060] The result of addition (the expression 14) grows N times of asignal of a single channel when the phase of each channel is coincident.N signal lines from the transducer 1 to the adder 6 are converted to onesignal line after the adder 6.

[0061] In the result of the addition (the expression), the term of exp(−j(ω_(s)−ω_(m))t) generally called a carrier component is left. As thecarrier component is not required for the reconfiguration of tomographicimages, the result of the addition (the expression 14) and the digitalreference signal k_(n) (t) expressed by the expression 15 are multipliedin the second mixer 11 in the invention.

[0062] As each processing in the analog to digital converter 2, thefirst mixer 10, the filter 4 and the digital delay unit 5 in the abovedescription is executed every channel, parallel N pieces of respectiveunits described above are required.

[0063] As N pieces of signals from the transducer 1 to the adder 6 areconverted to one signal in the adder 6, only one second mixer 11 hasonly to be provided. A product of the multiplication of the expression14 and the expression 15 is expressed by an expression 16.$\begin{matrix}\begin{matrix}{ {{k_{n}(t)} = {\exp( {{j( {\omega_{s} - \omega_{m}} )}t} }} \} \quad} \\\quad\end{matrix} & {{Expression}\quad 15} \\\begin{matrix}{  {{U(t)} = {N \times {A(t)} \times \{ {\exp( {{- {j( {\omega_{s} - \omega_{m}} )}}t} } }} \rbrack \} \times \{ {\exp ( {{j\lbrack {\omega_{s} - \omega_{m}} \rbrack}t} )} \}} \\{= {N \times {A(t)}}}\end{matrix} & {{Expression}\quad 16}\end{matrix}$

[0064] The absolute value of the product of the multiplication (theexpression 16) is acquired by the envelope detector 7. As in theultrasonic diagnostic apparatus according to the prior art shown in FIG.2, the scan converter 8 applies signal processing such as thecompression of a logarithm and gamma conversion to a signal output fromthe envelope detector 7. A signal output from the scan converter 8 isdisplayed on the display 9 as a tomographic image of an object to beinspected. In the expression 16, a complex number is generally acquiredand the envelope detector 7 calculates the absolute value of the complexnumber (the root sum square of a real part and an imaginary part).

[0065] Next, it will be described that in the configuration describedabove and shown in FIG. 1, even if the center frequency of the receivedsignal to be imaged is varied, the pass band of the filter 4 is notrequired to be varied.

[0066]FIGS. 3a to 3 c and 4 are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus according to the prior art shown in FIG. 2. FIG. 3a shows theskeletal form of a spectrum of a received signal expressed in theexpression 8 in case the center frequency ω_(s) is 3 MHz and the shapeof an envelope A (t) is Hanning window. In FIGS. 3 to 8, the axis ofordinates denotes spectral strength and the axis of abscissas denotes afrequency.

[0067] As a received signal expressed in the expression 8 is a realsignal, the spectral shapes are symmetrical based upon the axis ofordinates. A spectrum of a received signal to be imaged is a spectrumacquired by moving the central position of either spectrum having thecenter at a frequency −3 MHz or 3 MHz to a frequency zero.

[0068] In the ultrasonic diagnostic apparatus according to the prior artshown in FIG. 2, the multiplication expressed in the expression 5 incase 3 MHz is substituted for ω_(s) in the expression 4 is executed. Asa result of the multiplication, the spectrum shown in FIG. 3a is asshown in FIG. 3b. As shown in FIG. 3b, the central position of thespectrum of the received signal to be imaged is moved to the frequencyzero and the central position of an unnecessary spectrum is moved to afrequency 6 MHz.

[0069] Next, a pass band for extracting a signal between the frequency−3 MHz and 3 MHz is applied to the filter 4 as in an example shown inFIG. 3b, the required spectrum is extracted and a spectrum shown in FIG.3c is acquired. FIG. 3c shows the spectrum itself of the received signalto be imaged.

[0070]FIG. 4a shows the skeletal form of a spectrum of the receivedsignal expressed in the expression 8 in case the center frequency ω_(s)is 2 MHz and the shape of an envelope A (t) is Hanning window. Aspectrum of the received signal to be imaged is a spectrum acquired bymoving the central position of either spectrum having the center at afrequency −2 MHz or 2 MHz to the frequency zero.

[0071] In the ultrasonic diagnostic apparatus according to the prior artshown in FIG. 2, multiplication expressed in the expression 5 in case 2MHz is substituted for ω_(s) in the expression 4 is executed. As aresult of the multiplication, the spectrum shown in FIG. 4a is as shownin FIG. 4b. As shown in FIG. 4b, the central position of a spectrum ofthe received signal to be imaged is moved to the frequency zero and thecentral position of an unnecessary spectrum is moved to a frequency 4MHz.

[0072] Next, a pass band for extracting a signal between the frequency−2 MHz and 2 MHz is applied to the filter 4 as in an example shown inFIG. 4b, the required spectrum is extracted and a spectrum shown in FIG.4c is acquired. FIG. 4c shows the spectrum itself of the received signalto be imaged.

[0073] When the pass band shown in FIG. 4b of the filter 4 is the sameas the pass band shown in FIG. 3B of the filter 4, the unnecessaryspectrum having the center at the frequency 4 MHz cannot be completelyremoved and the quality of the image is deteriorated.

[0074] Therefore, it is clarified by the comparison of FIG. 3b and FIG.4b that in the ultrasonic diagnostic apparatus according to the priorart shown in FIG. 2, it is a requirement to vary the pass band of thefilter 4 corresponding to the center frequency of a received signal tobe imaged.

[0075] As the filter the frequency domain of a spectrum extracted bywhich is narrow has multiple taps as in the example shown in FIG. 4b,the scale of the circuit is larger than that of the filter having thepass band shown in FIG. 3b.

[0076] In the configuration of the ultrasonic diagnostic apparatusequivalent to the first embodiment of the invention, even if a receivedsignal has the spectrum shown in FIG. 4a, the pass band of the filter 4can be equalized to that shown in FIG. 3b.

[0077]FIGS. 5a to 5 d are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus equivalent to the first embodiment of the invention. FIG. 5ashows the skeletal form of a spectrum of an ultrasonic signal in casethe center frequency ω_(s) is 2 MHz and the shape of an envelope A (t)is Hanning window. In the invention, the multiplication expressed by theexpression 11 in case 3 MHz is substituted for ω_(m) in the expression10 is executed. As a result of the multiplication, a spectrum shown inFIG. 5b is acquired.

[0078] As shown in FIG. 5b, the central position of a spectrum of areceived signal to be imaged is moved to a frequency 1 MHz and thecentral position of an unnecessary spectrum is moved to a frequency 5MHz. A pass band for extracting a signal between the frequency −3 MHzand 3 MHz is applied to the filter 4 as in FIG. 3b, the requiredspectrum is extracted and a spectrum shown in FIG. 5c is acquired.

[0079] As shown in FIG. 5c, the spectrum the center of which is locatedat the frequency 1 MHz is left and a spectrum the center of which islocated at the frequency 5 MHz is removed. The spectrum shown in FIG. 5cis a spectrum of a signal output from the adder 6.

[0080] The center of a spectrum of an ultrasonic signal to be imaged isrequired to be located at the frequency zero as shown in FIG. 4c and thespectrum shown in FIG. 5c cannot be imaged as it is.

[0081] Then, in the invention, the multiplication expressed by theexpression 16 in case 2 MHz is substituted for ω_(s) in the expression15, that is, (ω_(s)−ω_(m))=−1 MHz is executed. As a result of themultiplication, the spectrum shown in FIG. 5C becomes a spectrum shownin FIG. 5D and the spectrum shown in FIG. 5d is the same as the spectrumto be imaged shown in FIG. 4c.

[0082] In the configuration of the ultrasonic diagnostic apparatusequivalent to the first embodiment of the invention, in case a receivedsignal the center frequency of which is 3 MHz is imaged, themultiplication expressed by the expression 11 in case 3 MHz issubstituted for ω_(s) and 3 MHz is substituted for ω_(m) in theexpression 10 is executed. As a result of the multiplication, thespectrum is the same as the spectrum shown in FIG. 3b.

[0083] As shown in FIG. 3b, the center of the spectrum of the receivedsignal to be imaged is moved to the frequency zero and the center of anunnecessary spectrum is moved to the frequency 6 MHz. A pass band forextracting a signal between the frequency −3 MHz and 3 MHz is applied tothe filter 4, the required spectrum is extracted and the spectrum whichis a spectrum of a received signal to be imaged itself and is the sameas the spectrum shown in FIG. 3c is acquired.

[0084] As described above, in the configuration of the ultrasonicdiagnostic apparatus equivalent to the first embodiment of theinvention, received signals the respective center frequencies of whichare 3 MHz and 2 MHz can be imaged without changing the pass band of thefilter 4. Also as described above, for the second mixer 11, one has onlyto be provided. As the configuration of the filter 4 can be simplifiedby adding one mixer circuit, the addition of one mixer circuit greatlycontributes to the miniaturization and the reduction of the price of theapparatus.

Second Embodiment

[0085]FIG. 6 shows an example of the configuration of an ultrasonicdiagnostic apparatus equivalent to a second embodiment of the invention.In detail, FIG. 6 shows the example of the configuration of theultrasonic diagnostic apparatus that simultaneously images and displaysplural received signals different in a center frequency without changingeach configuration of the analog to digital converter 2, the mixer 3,the filter 4 and the digital delay unit 5 respectively required everychannel in the configuration of the ultrasonic diagnostic apparatusshown in FIG. 1.

[0086] The configuration of the analog to digital converter 2, the firstmixer 10, the filter 4 and the digital delay unit 5 respectivelyrequired every channel in the configuration of the ultrasonic diagnosticapparatus shown in FIG. 6 is the same as that of the ultrasonicdiagnostic apparatus shown in FIG. 1. The configuration of theultrasonic diagnostic apparatus shown in FIG. 6 is based upon that ofthe ultrasonic diagnostic apparatus shown in FIG. 1 and is characterizedin that first and second signal processing circuits for executing signalprocessing succeeding second mixers 11 are provided in parallel andsecond filters 12 are provided between each second mixer 11 and eachenvelope detector 7 in the first and second signal processing circuits.

[0087] The first signal processing circuit is composed of the secondmixer 11 a, the second filter 12 a, the envelope detector 7 a, the scanconverter 8 a and the display 9 a, and the second signal processingcircuit is composed of the second mixer 11 b, the second filter 12 b,the envelope detector 7 b, the scan converter 8 b and the display 9 b.

[0088] The configuration of the ultrasonic diagnostic apparatus shown inFIG. 6 shows the effect in harmonic imaging for example. Harmonicimaging means a method of sending an ultrasonic signal of a frequency f₀and imaging a harmonic component (for example, a component of 2f₀) ofits received signal. According to this method, acoustic S/N can begreatly improved, compared with normal imaging in which a fundamentalwave component (a component of f₀) of a received signal is imaged.

[0089]FIGS. 7a to 7 c and 8 are explanatory drawings for explaining thespectral strength of an ultrasonic signal in the ultrasonic diagnosticapparatus equivalent to the second embodiment of the invention. FIG. 7ashows the spectrum of a received signal of a frequency 2 MHz alsoincluding a component of 4 MHz which is a harmonic component. Thespectral strength of the harmonic component is smaller than that of afundamental wave component.

[0090] In the ultrasonic diagnostic apparatus according to the prior artshown in FIG. 2, in case a received signal is imaged based upon itsfundamental wave component, ω_(s) becomes 2 MHz according to theexpression 4. In case a received signal is imaged based upon itsharmonic component, ω_(s) becomes 4 MHz according to the expression 4.However, according to the ultrasonic diagnostic apparatus according tothe prior art shown in FIG. 2, an image based upon its fundamental wavecomponent of a received signal and an image based upon its harmoniccomponent of the received signal cannot be simultaneously processed anddisplayed.

[0091] To simultaneously process and display an image based upon itsfundamental wave component of a received signal and an image based uponits harmonic component of the received signal, it is considered thatfirst and second signal processing circuits for executing signalprocessing succeeding the mixer 3 are provided in parallel in theultrasonic diagnostic apparatus according to the prior art shown in FIG.2 and each signal processing circuit is composed of the mixer 3, thefilter 4, the digital delay unit 5, the adder 6, the envelope detector7, the scan converter 8 and the display 9.

[0092] When multiplication in which 2 MHz is substituted for ω_(s) isexecuted by the mixer in the first signal processing circuit andmultiplication in which 4 MHz is substituted for ω_(s) is executed bythe mixer in the second signal processing circuit, an image based uponits fundamental wave component of a received signal and an image basedupon its harmonic component of the received signal can be simultaneouslyimaged. However, as the mixer 3, the filter 4 and the digital delay unit5 are required to be provided by the number of channels, the scale ofthe circuit becomes enormous.

[0093] In the configuration of the ultrasonic diagnostic apparatus shownin FIG. 6, multiplication expressed by the expression 11 in case 3 MHzis substituted for ω_(m) which is the mean frequency of the fundamentalwave 2 MHz of a received signal and the higher harmonics 4 MHz of thereceived signal in the expression 10 is executed. As a result of themultiplication, the spectrum shown in FIG. 7a becomes a spectrum shownin FIG. 7b. As shown in FIG. 7b, each center of spectrums of thereceived signal to be imaged is moved to frequencies −1 MHz and 1 MHzand each center of unnecessary spectrums is moved to frequencies 5 MHzand 7 MHz.

[0094] In case the fundamental wave component of the received signal isimaged, the spectrum having the center at the frequency 1 MHz isrequired to be left and in case the harmonic component of the receivedsignal is imaged, the spectrum having the center at the frequency −1 MHzis required to be left. As the mean value of frequencies of thefundamental wave of the received signal and higher harmonics of thereceived signal is used for a mixing frequency (ω_(m)=3 MHz), respectivecenter frequencies of the spectra of two received signal to be imagedare symmetrical based upon the axis of ordinates.

[0095] As shown in FIG. 7b, the same pass band as that in FIG. 3b isapplied to the filter 4. The spectrum of a signal output from the filter4 is as shown in FIG. 7c. As a delay process and an adding process donot change the shape of a spectrum, the spectrum of a signal output froman adder 6 is also the same as that shown in FIG. 7c.

[0096] Next, the second mixers 11 a and 11 b provided in parallelseparately execute shift in a frequency. The second mixer 11 amultiplies the expression 14 and the expression 15 in which 4 MHz issubstituted for ω_(s) and 1 MHz is substituted for (ω_(s)−ω_(m)). As aresult of the multiplication, the spectrum is as shown in FIG. 8A.

[0097] A pass band shown in FIG. 8a is applied to the second filter 12 aand only the spectrum of a harmonic component is passed. That is, thespectrum having the center at the frequency zero is left. FIG. 8b showsthe spectrum of a signal output from the second filter 12 a.

[0098] The second mixer 11 b multiplies the expression 14 and theexpression 15 in which 2 MHz is substituted for ω_(s) and −1 MHz issubstituted for (ω_(s)−ω_(m)) . As a result of the multiplication, thespectrum is as shown in FIG. 8c.

[0099] A pass band shown in FIG. 8c is applied to the second filter 12 band only the spectrum of the fundamental wave component of the receivedsignal is passed. That is, the spectrum having the center at thefrequency zero is left. FIG. 8d shows the spectrum of a signal outputfrom the second filter 12 b.

[0100] As filtering is executed for the signal output from the adder 6,two second filters 12 a and 12 b as a whole have only to be provided andeven if the scale of the filter circuit is expanded, it does not have alarge effect upon the price and the scale of the whole apparatus.

[0101] Each operation of the envelope detectors 7 a and 7b, the scanconverters 8 a and 8 b and the displays 9 a and 9 b is the same as thatin the first embodiment shown in FIG. 1. Signals output from the scanconverters 8 a and 8 b can be also displayed on one display.

[0102] In the above description, in case the image of the fundamentalwave component of the received signal and the image of the harmoniccomponent of the received signal are simultaneously imaged anddisplayed, the mean frequency of frequencies of the fundamental wave ofthe received signal and the higher harmonics of the received signal issubstituted for ω_(m). As long as the pass band of the filter 4 shown inFIG. 7b is varied even if ω_(m) in the expression 10 is an arbitraryfrequency and the frequency of the fundamental wave of the receivedsignal or the frequency of the higher harmonics of the received signalis substituted for ω_(s) in the expression 15, the expression 16 isunchanged, and the image of the fundamental wave component of thereceived signal and the image of the harmonic component of the receivedsignal can be simultaneously imaged.

[0103] However, in an actual ultrasonic diagnostic apparatus, an amountof time delay by the digital delay unit 5 is discrete and the minimumunit is equal to sampling time in the analog to digital converter 2 forexample. That is, it is difficult to precisely delay a signal by τ_(n).

[0104] When an error is included in a delay, the expression 7 isunchanged, however, the expression 16 varies. This will be describedbelow.

[0105] In the ultrasonic diagnostic apparatus shown in FIG. 2, only anenvelope component is influenced by time delay in the expression 7. Asthe period of an envelope is long enough, compared with sampling time,an error below sampling time included in a delay does not matter and theexpression 7 is unchanged.

[0106] However, in the ultrasonic diagnostic apparatuses shown in FIGS.1 and 6, a carrier component is also influenced by time delay in theexpression 13. A period of a carrier is not long enough, compared withsampling time. Therefore, in case an error is included in a delay,first, the expression 13 varies.

[0107] An actual delay for the expression 12 is set to T_(n) and anerror of time delay is defined by an expression 17.

Δτ_(n) =T _(n)−τ_(n)  Expression 17

[0108] The digital delay unit 5 delays a signal expressed by theexpression 12 by T_(n) and multiplies it by exp (−jω_(m)τ_(n)). As theprecision of complex multiplication depends upon the precision ofcalculation by an arithmetic circuit, the precision can be sufficientlyfined independent of sampling time. “V_(n) (t)” expressed by theexpression 18 is output from the digital delay unit 5. $\begin{matrix}\begin{matrix}{{V_{n}(t)} = {{g_{n}( {t + T_{n}} )}{\exp ( {{- {j\omega}_{m}}\tau_{n}} )}}} \\{= {{A( {t - \tau_{n} + T_{n}} )} \times \exp \{ {- {j\lbrack {{( {\omega_{s} - \omega_{m}} )( {t + T_{n}} )} - \varphi_{n} + {\omega_{m}\tau_{n}}} \rbrack}} \}}} \\{= {{A( {t + {\Delta\tau}_{n}} )} \times \exp \{  {- {j\lbrack {{( {\omega_{s} - \omega_{m}} )t} + {\omega_{s}T_{n}} - {\omega_{m}T_{n}} - {\omega_{s}\tau_{n}} + {\omega_{m}\tau_{n}}} }} ) \}}} \\{=   {{A( {t + {\Delta\tau}_{n}} )} \times \{ {\exp \lbrack {{- {j( {\omega_{s} - \omega_{m}} )}}t} \rbrack} \} \{ {\exp \{ {{- {j( {\omega_{s} - \omega_{m}} )}}{\Delta\tau}_{n}} } } \rbrack \}}\end{matrix} & {{Expression}\quad 18}\end{matrix}$

[0109] As the period of an envelope is long enough, compared withsampling time, the product of the expression 18 approximates that of anexpression 19 when the expression 19 is met.

A(t+Δτ_(n))=A(t)  Expression 19

[0110] However, Δτ_(n) in the term exp cannot be ignored. As Δτ_(n) isdifferent every channel, the result of the addition of V_(n) (t) everychannel of n pieces of elements used for one sending/receiving of anultrasonic wave by the adder 6 is S (t) shown in the expression 20.

S(t)=A(t)×{exp[−j(ω_(s)−ω_(m))t]}×{Σexp[−j(ω_(s)−ω_(m))Δτ_(n)]}  Expression 20

[0111] In the expressions 20 and 21, an adding symbol Σ denotes additionevery channel. The second mixer 11 executes complex mixing with thedigital reference signal k_(n) (t) expressed in the expression 11. Theresult of the multiplication of the expression 14 and the expression 20is U (t) expressed by an expression 21.

U(t)=A(t)×{Σexp[−j(ω_(s)−ω_(m))Δτ_(n)]}  Expression 21

[0112] That is, in case an error Δτ_(n) is included in a delay, theexpression 16 is equal to the expression 21. For example, if thesampling time of the analog to digital converter 2 is 40 ns and anactual delay T_(n) is equivalent to an integral multiple of the samplingtime, the absolute value of Δτ_(n) is 20 ns maximum.

[0113] If ω_(m)=3 MHz, (ω_(s)−ω_(m)) Δτ_(n) can be 1/50 wavelengthmaximum in case both ω_(s)=2 MHz and ω_(s)=4 MHz. Generally, as(ω_(s)−ω_(m)) Δτ_(n) may be ignored in forming a beam of an ultrasonicwave if it is 1/32 wavelength or less, an error in the expression 21 isin a range in which the error may be ignored.

[0114] However, if ω_(s)=2 MHz in case ω_(m)=2.2 MHz, (ω_(s)−ω_(m))Δτ_(n) is 1/250 wavelength maximum, however, if ω_(s)=4 MHz,(ω_(s)−ω_(m)) Δτ_(n) is 1/28 wavelength maximum. In this case, for thefundamental wave component of a received signal, the error in theexpression 21 may be ignored, however, for the harmonic component of thereceived signal, the error in the expression 21 cannot be ignored.

[0115] That is, there is an advantage that when the mean value of thefrequency of a fundamental wave of a received signal and the frequencyof higher harmonics of the received signal is substituted for ω_(m), theerror in the expression 21 can be reduced in any frequency.

[0116] In case two different frequency components of a received signalare imaged and are simultaneously displayed, the two frequencycomponents are not limited to a fundamental wave of the received signaland higher harmonics of the received signal and can be set to arbitrarypredetermined two frequencies.

INDUSTRIAL APPLICABILITY

[0117] As described above, according to the invention, received signalsdifferent in a center frequency can be imaged without changing the passband of the filter.

[0118] Further, signal processing for acquiring plural images differentin the center frequency of the received signal can be simultaneouslyexecuted in parallel, even if the center frequency of the receivedsignal to be imaged is varied, the pass band of the filter every channelis not required to be varied and the digital ultrasonic diagnosticapparatus by which a high quality of tomographic image can be acquiredcan be realized.

1. An ultrasonic diagnostic apparatus, comprising: a transducer composedof plural elements that send an ultrasonic wave to an object to beinspected and receive the ultrasonic wave reflected from the object tobe inspected; an analog to digital converter that digitizes each ofplural received signals received by the plural elements; a first mixerthat multiplies a signal output from the analog to digital converter anda first digital reference signal; a first filter that extracts a signalhaving a predetermined center frequency from a signal output from thefirst mixer; a digital delay unit that delays a signal output from thefirst filter; an adder that adds plural signals output from each digitaldelay unit; a second mixer that multiplies a signal output from theadder and a second digital reference signal; an envelope detector thatdetects a signal output from the second mixer; a scan converter thatconverts a signal output from the envelope detector to a picture signal;and a display that displays a signal output from the scan converter. 2.An ultrasonic diagnostic apparatus, comprising: a transducer composed ofplural elements that send an ultrasonic wave to an object to beinspected and receive the ultrasonic wave reflected from the object tobe inspected; an analog to digital converter that digitizes each ofplural received signals received by the plural elements; a first mixerthat multiplies a signal output from the analog to digital converter anda first digital reference signal; a first filter that extracts a signalhaving a predetermined center frequency from a signal output from thefirst mixer; a digital delay unit that delays a signal output from thefirst filter; an adder that that adds plural signals output from thedigital delay units; and plural signal processing circuits connected tothe output terminal of the adder in parallel, wherein: each signalprocessing circuit comprises: a second mixer that multiplies a signaloutput from the adder and a second digital reference signal; a secondfilter that extracts a signal having a predetermined center frequencyfrom a signal output from the second mixer; an envelope detector thatdetects a signal output from the second filter; and a scan converterthat converts a signal output from the envelope detector to a picturesignal; and in each signal processing circuit, the second filterextracts a signal having a predetermined center frequency differentevery signal processing circuit from the signal output from the secondmixer.
 3. An ultrasonic diagnostic apparatus according to claim 2,wherein: the signal processing circuit is composed of a first signalprocessing circuit and a second signal processing circuit; the firstsignal processing circuit and the second signal processing circuit arerespectively composed of a second mixer that multiplies a signal outputfrom the adder and a second digital reference signal, a second filterthat extracts a signal having a predetermined center frequency from asignal output from the second mixer, an envelope detector that detects asignal output from the second filter and a scan converter that convertsa signal output from the envelope detector to a picture signal; in thefirst and second signal processing circuits, each second filter extractsa signal having a predetermined center frequency different between thefirst and second signal processing circuits from a signal output fromthe second mixer; and signal processing for acquiring images of a firstcenter frequency and a second center frequency is executed in parallel.4. An ultrasonic diagnostic apparatus according to claim 1 or 2,wherein: the sum of the frequency of the first digital reference signaland the frequency of the second digital reference signal is equalized tothe center frequency of a received signal to be imaged.
 5. An ultrasonicdiagnostic apparatus according to claim 2, wherein: the images havingdifferent center frequencies are displayed on the same display or ondifferent displays.
 6. An ultrasonic diagnostic apparatus according toclaim 4, wherein: the frequency of the first digital reference signal isset between the first center frequency and the second center frequency.7. An ultrasonic diagnostic apparatus, comprising: a transducer composedof plural elements; a selecting/driving circuit that selects plural npieces of elements (n=1, 2, - - - , N) for sending an ultrasonic wavehaving a center frequency ω_(s) to an object to be inspected andreceiving the ultrasonic wave having the center frequency ω_(s)reflected from the object to be inspected; analog to digital convertersprovided corresponding to the plural n pieces of elements (n=1, 2, - - -, N) for digitizing a received signal received by the element; firstmixers provided corresponding to the analog to digital converterscorresponding to the elements for multiplying a signal output from eachanalog to digital converter and a first digital reference signal havinga frequency ω_(m) different from the center frequency ω_(s); firstfilters provided corresponding to the first mixers corresponding to theelements for extracting a signal having a frequency (ω_(m)−ω_(s)) from asignal output from each first mixer; digital delay units providedcorresponding to the first filters corresponding to the elements formultiplying a signal output from each first filter and delayed bypropagation delay time τ_(n) different every element from the sending ofthe ultrasonic wave to the receiving of it by exp (−jω_(m)τ_(n)); anadder corresponding to the plural n pieces of elements (n=1, 2, - - - ,N) for adding signals output from the digital delay units; a secondmixer that multiplies a signal output from the adder and a seconddigital reference signal having a frequency (ω_(s)−ω_(m)); an envelopedetector that detects a signal output from the second mixer; a scanconverter that converts a signal output from the envelope detector to apicture signal; and a display that displays a signal output from thescan converter, wherein: a received signal having the center frequencyω_(s) is imaged and displayed.
 8. A method of processing an ultrasonicsignal, comprising: (1) a process for selecting plural n pieces ofelements (n=1, 2, - - - , N) of a transducer composed of plural elementsfor sending an ultrasonic wave having a center frequency ω_(s) to anobject to be inspected and receiving the ultrasonic wave reflected fromthe object to be inspected and having the center frequency ω_(s) anddriving them; (2) a process for digitizing plural received signalsreceived by the elements in analog to digital converters providedcorresponding to the plural n pieces of elements (n=1, 2, - - - , N);(3) a process for multiplying a signal output from each analog todigital converter and a first digital reference signal having afrequency ω_(m) different from the center frequency ω_(s) in the firstmixers provided corresponding to the analog to digital converterscorresponding to the elements; (4) a process for extracting a signalhaving a frequency (ω_(m)−ω_(s)) from a signal output from each firstmixer in first filters provided corresponding to the first mixerscorresponding to the elements; (5) a process for multiplying a signalacquired by delaying a signal output from each first filter bypropagation delay time τ_(n) different every element from the sending ofthe ultrasonic wave to the receiving of it by exp (−jω_(m)τ_(n)) indigital delay units provided corresponding to the first filterscorresponding to the elements; (6) a process for adding signals outputfrom the digital delay units corresponding to the plural n pieces ofelements (n=1, 2, - - - , N) in an adder; (7) a process for multiplyinga signal output from the adder and a second digital reference signalhaving a frequency (ω_(s)−ω_(m)) in a second mixer; (8) a process fordetecting a signal output from the second mixer in an envelope detector;(9) a process for converting a signal output from the envelope detectorto a picture signal in a scan converter; and (10) a process fordisplaying a signal output from the scan converter on a display,wherein: the received signal having the center frequency ω_(s) is imagedand displayed.